Stable bromine isotopic composition of atmospheric CH3Br

Tropospheric methyl bromide (CH3Br) is the largest source of bromine to the stratosphere and plays an important role in ozone depletion. Here, the first stable bromine isotope composition (%26dgr%3B81Br) of atmospheric CH3Br is presented. The %26dgr%3B81Br of higher concentration Stockholm samples and free air subarctic Abisko samples suggest a source/background value of %26minus%3B0.04%26plusmn%3B0.28%26permil%3B ranging up to +1.75%26plusmn%3B0.12%26permil%3B. The Stockholm %26dgr%3B81Br versus concentration relationship corresponds to an apparent isotope enrichment factor of %26minus%3B4.7%26plusmn%3B3.7%26permil%3B, representing the combined reaction sink. This study demonstrates the scientific potential of atmospheric %26dgr%3B81Br measurements, which in the future may be combined with other isotope systems in a top-down inverse approach to further understand key source and sink processes of methyl bromide.